As discussed earlier in this chapter and in
chapter 6, various carbonaceous
solids are expected to form around carbon-rich AGB stars. So far, the
discussion has centred on silicon carbide, PAHs and various forms of
hydrogenated amorphous carbon. Another form of carbon that could be present
around these stars is diamond.

Diamonds in the interstellar medium were were first proposed by Saslaw & Gaustad
(1969), who argued that, although graphite is the thermodynamically stable
form of carbon in dust forming regions and in interstellar space, it was
possible that diamond could form as a metastable product(see
section 3.2.3).
Following this theoretical work, interest in interstellar diamonds lay dormant
until, in 1987, they were found, in meteorites, rather than in interstellar
spectra (Lewis et al., 1987).

Interstellar diamonds have been found in various carbonaceous chondrites.
Their extra-solar origin has been established in various ways. Firstly, objects
incorporated into meteorites must predate the meteorite, and chondrites
date from the formation of the solar system, therefore the presence of
diamonds within chondrites indicates that the diamonds predate the solar
system. The presolar diamonds were actually discovered whilst attempting to
find the source of an isotopically anomalous noble gas component in
carbonaceous chondrites. There was isotopic evidence that Xenon was enriched
by up to a factor of 2 in both the lightest and the heaviest isotopes compared to
the dominant xenon component in meteorites and xenon in the Earth's atmosphere.
This anomalous component is known as the Xe-HL component. This Xe-HL component
is only found in diamond inclusions and is not present in any other presolar
grain. Furthermore, it is found in all presolar diamond inclusions that have
been studied. The two parts to the Xe-HL component (the heavy and the light
isotopes) have proved to be completely inseparable in the laboratory. It is
clear that the anomalous xenon is an indicator that the meteoritic diamond is
extra-solar in origin. Physical details of these meteoritic presolar diamonds
can be found in section 3.2.3.

When meteoritic diamonds were discovered, there had been no observational
evidence for diamonds in interstellar space. Then, in 1992, Allamandola et
al. (1992) found an absorption feature in the spectra of protostars embedded in dense
molecular clouds at 3.47µm (2880 cm-1 ), which they attributed to
sp3 bonded C-H, i.e. hydrogenated diamond-like particles. Thus, we
have evidence of diamonds in space. Their observations suggested that the
diamond-like grains seemed to be ubiquitous in dense clouds, while methyl
(-CH3) and methylene (-CH2) rich material dominated the diffuse ISM.
The diamond signature was only found in dense molecular clouds and not in the
diffuse ISM, which was surprising because thorough mixing is expected to occur
between the dense and diffuse media. Allamandola et al. (1992) suggested
that the absence of -CH2 and -CH3 bands, together with appearance
of the sp3 bonded -CH not found in the diffuse ISM, implies that
C-rich materials in the diffuse medium do not become incorporated into, or do
not survive incorporation into, dense molecular clouds. Neither destruction by shocks
(which are weak in dense clouds) or UV photolysis (mild as a result of dust
extinction), or H atom attack (which would make -CH2 and -CH3
groups rather than destroy them) can explain the apparent lack of the
carbon-rich diffuse cloud components in dense clouds.

It is my proposal that we
need to turn the argument around. I would argue that, without any explanation of
how it got there, the sp3 bonded -CH is stable in the dense
molecular clouds where it is protected from the severe environment outside.
Only when it leaves the dense cloud is it converted into -CH2 and
-CH3 by the harsher environment. If this is the case, we need to seek out
a very effective mechanism by which -CH2 and -CH3 groups would be
converted into sp3 bonded -CH inside the dense molecular clouds.
However, if this paradigm is correct, the diamonds are formed in dense
molecular clouds and are not of interest in a discussion of dust formation
around carbon-rich AGB stars. Therefore, let us consider the meteoritic
evidence for the origins of interstellar diamonds further.

The isotopic complexity of the noble gas component in meteoritic diamonds
indicates that it comes from several sources. Other isotopically anomalous
elements have also been found, including barium and strontium, which are
slightly enriched in r-process isotopes. Nitrogen is also anomalous. The
most studied aspect of meteoritic diamond isotope anomalies is the noble gas
component. The noble gases are released by stepped heating of diamond residues
extracted from meteorites. This has revealed three components, each including
all five noble gases: 1) the roughly solar system component released between
200 and 900°C; 2) the HL component (anomalies in all noble gases) released
at 1100-1600°C; and 3) a mixture of the first two released at even higher
temperatures. Whether the carrier of the first and second components are
distinct phases has been the subject of some discussion. It seems that the
carrier of the solar system component is a more disordered carbon, with both
sp3 and sp2 bond C-H, known as a-C:H (amorphous hydrogenated
carbon). There are indications that this phase is lost through
metamorphism, since the diamonds in the less primitive meteorites do not have
this phase. It has been suggested that the carrier of the solar system
component is merely the hydrogenated surface of the diamonds, which is
supported by EELS (electron energy loss spectroscopy; Bernatowicz et al. 1990)
data. However,
the isotopic data implies that it formed at a different time or place to the
more anomalous diamond. it is possible that in the diffuse interstellar medium,
the diamonds acquire a coating of the a-C:H through hydrogenation of their
surfaces, rendering the diamonds invisible by changing their spectral
properties to those of methyl or methylene groups. Somehow this coating must
disappear when the grains enter the dense molecular clouds in order to fit
observations. Therefore a mechanism for the loss of the -CH2 and -CH3
coating needs to be established.

The origin of the meteoritic diamonds is still an enigma. The Xe-HL component
must have been formed near a supernova, since the enrichment of the lightest
and heaviest isotopes of xenon would proceed through the p-process and
r-process respectively, both of which are associated with supernovae.
This has led to various hypotheses for the formation of diamonds in space.

Firstly, there is the chemical vapour deposition (CVD) method proposed by Saslaw
& Gaustad (1969) and others (e.g. Anders & Zinner 1993 and references therein;
Colangeli et al. 1994). This is favoured by the size distribution
of meteoritic diamonds which is log-normal and is indicative of grain growth
rather than fragmentation of larger grains, which tends to give a power law
distribution. For this mechanism to be practicable, the diamonds must form in
an environment with a carbon-to-oxygen ratio greater than unity. However, the
isotope studies imply that a supernova is involved in the process. Carbon stars
are not massive enough to become Type II supernovae and the supernova
precursors are carbon poor. A possible mechanism, expounded by various authors (e.g. Lewis
et al. 1987), is that diamond grains form and grow around C-rich AGB stars and then
receive the noble gases by implantation in the vicinity of a supernova. I
believe this mechanism is somewhat implausible because this scenario would
imply that some diamonds would be formed around C-rich AGB stars and not
necessarily be exposed to supernovae noble gases. These grains should also have
been incorporated into the early solar system, a result which is not
substantiated by the isotopic evidence.

Another suggestion for the formation of diamond is the collision of graphitic
grains in supernova shocks (Tielens et al. 1987). As the efficiency of
this process is
estimated to be only ~5%, the diamonds should be accompanied by a
twenty-fold excess of unconverted graphitic carbon with the same isotopic
composition and the same noble gas components. However, there is no evidence of
such unconverted graphitic carbon in meteorites. The lack of graphitic carbon
cannot be blamed on preferential destruction in the early solar system, because
deuterium-rich, and therefore interstellar, organic carbon survived in the same
meteorites, despite greater fragility. Even if the graphitic carbon was converted to
organic carbon through reaction with hydrogen in the ISM, this would also produce an
unseen twenty-fold increase in the amount of organic carbon. Furthermore, work by
Daulton et
al. (1996) has given persuasive evidence for a low pressure mechanism based
on high-resolution TEM studies. Their nanostructural comparison of meteoritic
and synthetic diamond crystallites strongly favours a CVD-like process as
opposed to one involving high pressure, shock induced metamorphism of
pre-existing carbonaceous material.

Jørgensen (1988) tried to solve this problem by invoking a binary carbon
star system. Matter flows to the more massive star after it becomes a white
dwarf, permitting it to explode as a Type I supernova. Xe-HL ions in the high
speed ejecta overtake the diamond dust shell produced during the red giant
phase and implant themselves in the diamonds. Clayton (1989), on the other
hand, proposed that Xe-HL is made in Type II supernova by neutrino-produced
neutrons in the helium shell, and diamonds condense from the expanding shell
about a year after the explosion, trapping the ambient Xe-HL. However, this
theory only accounts for the heavy xenon component. The heavy and light xenon
components have been inseparable in the laboratory, which led Manuel et
al. (1972) to argue that the p- and r-process nuclei must have
been mixed in the gas phase before Xe-HL was incorporated into the diamonds.
There have been several major objections to the theories of both Jørgensen
(1988) and Clayton (1989; e.g. Lewis et al. 1989), which are beyond the
scope of the present work, but which are taken as negating these mechanisms for
the formation of meteoritic diamonds.

Nuth & Allen (1992) invoked a supernova for the Xe-HL, but suggested that the diamonds
were made from pre-existing carbonaceous dust rather than supernova or red-giant
material. They proposed that small (<100Å) hydrocarbon grains in the
vicinity of a supernova are ``annealed'' to diamond by absorption of several far
ultraviolet photons, lose all their pre-existing gases and then trap heavy
ions and neutral atoms from the supernova ejecta. However, this quantum
heating may be a problem: such ``annealing'' of carbonaceous grains requires
temperatures of over 1000K. Since the energy of the photon is distributed over
the entire grain, such temperatures are only achievable by grains of <90 atoms
(Anders & Zinner 1993). It is not clear
whether this mechanism can yield diamonds larger than 90 atoms, which
comprises the bulk of the size distribution of the diamonds in meteorites.

Another proposed mechanism for diamond formation uses photolysis of hydrocarbons
(Buerki & Leutwyler 1991). Ethene (C2H4) and mixtures of ethene, molecular
hydrogen (H2) and silane (SiH4) have been decomposed using a laser to obtain
spherules of cubic and hexagonal diamonds along with PAHs, organic polymers,
graphite and amorphous carbon. The diamond spherules formed had a mean size
ranging from 63±24 to 1200±240Å. However, it is not yet known whether
this process would work under astrophysically relevant conditions, i.e. lower
photon fluxes, lower pressures and higher H/C ratios. If it was a viable
formation mechanism, then the range of possible diamond formation sites would
be greatly expanded.

At present it seems that the CVD method for producing diamonds is most favoured
(Saslaw & Gaustad 1969; Wright 1992; Lewis et al. 1989; Daulton et al.
1996), although detailed mechanisms by which the diamonds obtain their
isotopically anomalous noble gases are yet to be understood. This suggests that
diamond production around C-rich AGB stars is not unfeasible, and is possibly
expected.

There have been various attempts to get representative spectra from meteoritic
diamonds from several different meteorites (Murchison, Allende, Orgeuil; Lewis
et al. 1989; Koike et al. 1995b; Mutschke et al. 1995; Hill
et al. 1997; Andersen et al. 1998). A summary of the features seen in
these spectra can be seen in Table 7.5. All of these spectra seem to
be entirely different from one another. Only one of them shows the 3.47µm
(2880 cm-1 ) band seen in the spectra of dense molecular clouds (Hill et al. 1997).
In fact Mutschke et al. (1995) have discredited many of these spectra, claiming
that many of the features are artifacts of the extractions and spectroscopic
techniques.

The most recently published meteoritic diamond spectrum can be seen in
Fig. 7.5, which shows the entire spectrum from 2.5 to 25µm,
and Fig. 7.6 which show the 7.5-13.5µm region of the spectrum,
relevant to our observations. This spectrum comes from Andersen et al.
(1998), who use their newly measured optical properties of meteoritic diamond in a
model of the stellar atmosphere of a carbon star. They suggest that diamonds
form in the atmospheres of carbon stars and act as nucleation seeds for other
dust grains. Hitherto, PAHs were expected to act as nucleation centres. However,
Andersen et al. (1998) found that the timescales for PAH formation are
too long compared with the dynamical timescales and the gas temperature is too
high for PAH formation. The relatively modest opacity and higher condensation
temperature of diamond may cause nucleation of diamond grains at relatively
high atmospheric temperatures where the velocity field is still negligible.

Given that there are various models for the formation of diamonds around C-rich
AGB stars, it would appear that we need to be looking for evidence of diamonds
around such stars. The diamond spectrum from Andersen et al. (1998), shown
in Figs. 7.5 & 7.6, shows various features which
may be detectable in the infrared spectra of C-rich AGB stars. It is not yet
clear what environmental ingredients are necessary to induce the IR emission in
the diamond grains, however, even if they require ultraviolet radiation to emit
in this region, we have already shown that C-rich AGB stars in binary systems
could fulfil this requirement (see section 7.2). The meteoritic diamond
spectrum shown in Fig. 7.5 covers a relatively large wavelength
range (2.5-25µm), and the majority of the features are beyond the
wavelength range of our observational spectra. However, as seen in
Fig. 7.6, there is a prominent feature at ~8.5-9.5µm
which may be detectable in the carbon stars in our sample. Unfortunately this
is in the same position as the hypothesised a-C:H (see section 6.2.3), making
it difficult to conclusively attribute the feature in the observed spectra to
diamond grains. It would be interesting to study the high-resolution, wider
wavelength range, ISO-SWS spectra of C-rich AGB stars with a view to matching
all the features seen in Fig. 7.5 to features in carbon star
spectra.

At present our observations have too limited a wavelength range to
satisfactorily investigate the possibility of diamond grain formation around
carbon stars.